This invention relates to a spring balance mechanism for manhole covers. Most manhole covers on tanks, vessels and other processing equipment are hinged. Manhole covers have often caused injury to operators and damage to the assembly by falling back on the manhole collar, particularly the ones on glass lined vessels. Manhole covers on pressure vessels are usually very heavy and hard to operate, especially for women, who are being employed in considerably larger numbers in the processing and heavy industries. By use of spring balance devices for manhole covers, a manhole cover can be opened and closed more easily and safely than a manhole cover without balance assistance, and the possibility of injury from falling covers is virtually eliminated.
One such system, shown in German Gebrauchsmuster No. 7,402,411, uses a set of conical disc springs which are compressed as the cover closes. This system is more desirable than ones with helical springs because it is more compact. However, the system also has certain disadvantages.
The system described in the reference has a regressive or diminishing spring force curve. In other words, the incremental change in the force exerted, for the corresponding incremental compression of the springs, decreases as the springs are compressed. For the geometry of most manhole cover mechanisms, the spring force curve should be progressive, not regressive, to more nearly match the force required to balance the manhole in any position. However, almost all commercially available disc springs have a regressive spring force characteristic.
Another disadvantage of the system shown in the reference is that there is no provision to absorb shocks which can occur if the cover is opened rapidly, which is not unlikely with a balancing system that allows the cover to be moved easily. Hinged mounting systems such as the one shown in the reference typically include stops to limit the opening movement of the cover. If the cover is swung rapidly to the point where stops limit the movement, the shock of the sudden deceleration of the cover movement must be absorbed by the mounting system. High stresses can be created in this situation.
Yet another disadvantage of the system shown in the reference is that it must be attached to the vessel itself by drilling or welding, which makes it difficult to install on existing glass lined vessels because of the dangers of damaging the glass lining, and the dangers of explosion created by working on vessels in hazardous environments.